In order to saving energy and emission reduction, a secondary balance technology was used to reform the conventional beam pumping unit. The output torque from the reduction gearbox of conventional beam pumping units is usually characterized by its periodic drastic changes. Based on the idea of "cutting peak and filling valley" and the theory of the Fourier series expansion for torque curve, the second balance device is designed in order to slow down the fluctuations in the torque curve. The second balance device, with the similar balance principle to the crank balance, is connected to the output shaft of the reduction gearbox and can further reduce torque fluctuation rate and peak torque. Field test result of the secondary balance device shows that the Root-Mean-Square (RMS) torque is decreased by 22.7% and the energy-saving rate of motor reaches 6.54%.